[PATCH] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid()

[ghackmann@google.com (Greg Hackmann)]

On 08/14/2018 03:40 AM, Will Deacon wrote:
> Hi Greg,
> 
> On Mon, Aug 13, 2018 at 12:30:11PM -0700, Greg Hackmann wrote:
>> ARM64's pfn_valid() shifts away the upper PAGE_SHIFT bits of the input
>> before seeing if the PFN is valid.  This leads to false positives when
>> some of the upper bits are set, but the lower bits match a valid PFN.
>>
>> For example, the following userspace code looks up a bogus entry in
>> /proc/kpageflags:
>>
>>     int pagemap = open("/proc/self/pagemap", O_RDONLY);
>>     int pageflags = open("/proc/kpageflags", O_RDONLY);
>>     uint64_t pfn, val;
>>
>>     lseek64(pagemap, [...], SEEK_SET);
>>     read(pagemap, &pfn, sizeof(pfn));
>>     if (pfn & (1UL << 63)) {        /* valid PFN */
>>         pfn &= ((1UL << 55) - 1);   /* clear flag bits */
>>         pfn |= (1UL << 55);
>>         lseek64(pageflags, pfn * sizeof(uint64_t), SEEK_SET);
>>         read(pageflags, &val, sizeof(val));
>>     }
>>
>> On ARM64 this causes the userspace process to crash with SIGSEGV rather
>> than reading (1 << KPF_NOPAGE).  kpageflags_read() treats the offset as
>> valid, and stable_page_flags() will try to access an address between the
>> user and kernel address ranges.
>>
>> Signed-off-by: Greg Hackmann <ghackmann@google.com>
>> ---
>>  arch/arm64/mm/init.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> Thanks, this looks like a sensible fix to me. Do you think it warrants a
> CC stable?
> 
> Will

Yes, I think so.  Should I resend with a "Fixes" field?